Combine the flour, sugar, baking powder, and salt in a large mixing bowl and whisk together.
Add the grated butter, toss in the flour mixture, and then use your hands to break up the butter into pea size pieces.
Add the blackberries and toss to distribute evenly.
Make a well in the center of the dry ingredients, and in the well, add the sourdough starter, egg and egg yolk, vanilla, and milk. Mix the wet ingredients together and then stir them in with the dry ingredients until just combined, avoiding overmixing.
Shape the scone dough into an 8-inch round disk. Cut it into eight equal pie-shaped pieces, place on a parchment-lined baking sheet. Brush the tops of the unbaked scones with heavy cream and sprinkle with sugar.
Cover the scones (with plastic wrap or something air-tight) and allow to ferment in the refrigerator for 8-72 hours.
When ready to bake, preheat the oven to 400 degrees Fahrenheit. Remove the baking sheet from the refrigerator, uncover the scones, and bake for 20-25 minutes, or until lightly golden on the tops and edges. Remove and allow to cool for 5 minutes.